Remove Unreachable Code

[nixpulvis]

In an effort to start some simple optimizations, we could save a bunch of space in some brainfuck programs by determining up front that some while loops (aka [...] constructs) will never be entered, and can be removed.

A baseline argument could be if there has been no reachable user input, then we can calculate the values of each cell. This might be expensive, so maybe for a start we simply track readable values until the first reached loop.

This covers the most common case of using a loop for comments at the beginning of a file.